I can't draw good lines with the UV texture brush
You can set the brush parameters in the UV/Image Editor tools panel (T to display).

If you want a continuous line you should use the Space stroke method (you can also press E in the image window to switch form a stroke to another). 

Make sure that the Spacing value is low enough so that the line is not broken. 

To make the line sharp, change its curve (you can add or remove points to give the curve the shape you want), if you select the second one it is pretty sharp.

To save the brush you've created, on the top of the panel, on the right of the brush name, click on the "+" button.

Also good to know, in the image window, F to increase the Radius, shiftF to increase the Strength, X.